In recent years, silicon nitride masks have become a sought-after expedient in the fabrication of integrated circuits. Originally, the art applied masking layers comprising silicon nitride directly onto silicon substrates. This gave rise to problems associated with high stresses created on the underlying silicon substrate by the silicon nitride-silicon interface. Such stresses were found in many cases to produce dislocations in the silicon substrate which appear to result in undesirable leakage current pipes and otherwise adversely affect the electrical characteristics of the interface. In order to minimize such interface stresses with silicon nitride layers, it has become the practice in the art to form a thin layer of silicon dioxide between the silicon substrate and the silicon nitride layer. While this approach has been relatively effective in the cases where this silicon dioxide-silicon nitride composite is utilized only for passivation, problems have arisen where these silicon dioxide-silicon nitride composites have been utilized as masks, and, particularly, when utilized as masks against thermal oxidation. During such thermal oxidation, there is a substantial additional lateral penetration of silicon oxide from the thermal oxidation beneath the silicon nitride. This lateral penetration is greatest at the mask-substrate interface to provide a laterally sloping structure known and recognized in the prior art as the undesirable "bird's beak."
The publications, "Local Oxidation of Silicon; New Technological Aspects," by J. A. Appels et al, Phillips Research Report 26, pp. 157 - 165, June 1971, and "Selective Oxidation of Silicon and Its Device Application," E. Kooi et al, Semiconductor Silicon 1973, published by the Electrochemical Society, Edited by H. R. Huff and R. R. Burgess, pp. 860 - 879, are representative of the recognition in the prior art of the "bird's beak" problems associated with silicon dioxide-silicon nitride composite masks.
The "bird's beak" problems are particularly significant when silicon dioxide-silicon nitride composite masks are used in the formation of recessed silicon dioxide to be used for dielecric isolation. In such recessed oxide formation techniques, the silicon dioxide-silicon nitride composite masks are first used as an etch barrier while recesses are etched through the mask openings in the silicon substrate. These recesses are subsequently subjected to the previously described thermal oxidation to form recessed silicon dioxide regions providing dielectric isolation extending into the silicon substrate from the surface. Such recessed silicon dioxide regions would be most desirably coplanar with the remainder of the silicon surface. However, as a result of the "bird's beak," a lateral junction or edge of the recessed silicon dioxide isolation region is very vaguely defined. With any recessed oxide isolation it is highly desirable that the lateral edges of the recessed silicon dioxide be substantially vertical, i.e., perpendicular to the semiconductor substrate surface. Instead, as a result of the "bird's beak", the edges of the recessed silicon dioxide are gradually sloped with respect to the silicon surface, being at an angle which varies from 15.degree. to 30.degree. with respect to the surface instead of the desirable 90.degree. angle.
Because of this gradual lateral junction in the recessed silicon dioxide, the recessed area does not clearly define abutting regions introduced by either diffusion or ion implantation, particularly shallow abutting regions. In the case of such shallow abutting regions, there is a distinct possibility that during subsequent etching steps part of the "bird's beak" at the surface will be etched away to provide an undesirable exposure of the P-N or other junction of the abutting shallow region.
However, even with deeper regions formed by diffusion, the indefiniteness of the lateral junction of the abutting recessed silicon dioxide region renders it difficult to control lateral geometries of introduced region, and therefore imposes the need for wider tolerances of lateral dimension in the integrated circuit layout.
The above mentioned lack of definition because of the "bird's beak" is particularly pronounced when the recessed silicon dioxide regions abutting the silicon region are utilized to define a region of a given conductivity type introduced into a silicon substrate region adjoining such recessed silicon dioxide regions. In such a case, one of the significant advantages of recessed oxide technology as taught in the prior art is the ability to eliminate precise mask alignment steps when introducing said conductivity-type region. In accordance with the art, it is desirable to first cover the surface of the substrate with a layer of an insulative material, particularly silicon dioxide, after which a step involving only very gross masking coupled with dip etching is utilized to avoid such mask alignment when forming openings in the silicon dioxide layer through which the conductivity-determining impurities are to be introduced into the silicon substrate. The dip etching process is continued for a time calculated to be sufficient to remove only the deposited silicon dioxide layer from the surface of selected silicon substrate region (the selection of regions is of course determined by the gross block-out mask) but insufficient to affect the surrounding recessed silicon dioxide region. However, because of the "bird's beak," the extent of such surrounding recessed oxide regions, particularly at the substrate surface, becomes indefinite and the portion of the silicon substrate exposed may vary substantially dependent on the extent of the "bird's beak." Thus, because of the variation in opening size, the introduced region may vary substantially in lateral dimension.
Because of this variation of lateral dimensions, contact openings made to such introduced regions through subsequently formed insulative layers cannot be made with any definiteness or precision because such contact opening may expose a surface junction between the introduced region and an abutting region of semiconductor material. Accordingly, an additional advantage of recessed silicon dioxide technology, i.e., that of defining contact opening to abutting regions formed in the substrate is also unrealized.